There's not that much info to go on here. Sounds like you may be continuing from another thread?
If the horse is in good health & sound, 20yo is not much past her prime. Of course she won't become an athlete at that age, but there shouldn't be anything 'normal' she can't do just because of her age.
What do you WANT to do with her? Eg. If you're happy riding english, why go western? If you want to trail ride, work towards that - take her out & about on lead first. Work out what she 'hates' about trails - is it being without her herd, unconfident in strange environs...?
Re sidepassing, it may be pain, confusion, fear(from confusion), but it's not likely to be anger.